In many turbidimeters and nephelometers known in the art a sample is illuminated through a window. A problem occurring through this is, that the window may be contaminated and accordingly loose (some of) its transparency. It is not easy to distinguish the corresponding loss of light intensity from the effect of a different turbidity of the sample.
A free-fall arrangement is also known from the art: a sample liquid forming a free-forming jet is illuminated by light, and scattered light is detected. In such arrangements, windows are not needed, but usually a large amount of sample liquid is needed for measurements.
From U.S. Pat. No. 5,400,137 a photometric means is known, which allows to determine fluorescence and scattering of a sample liquid. Two light beams of different wavelengths are directed at a surface of said sample liquid, and a detector detects light emitted thereupon from said sample liquid. The sample surface is formed in a container and its boundary is formed by means of an edge to be overflown by excessive sample liquid.
In RU Patent 2,235,310 a turbidimeter is presented, in which a sample liquid contained in a vessel is illuminated through its surface by light directed parallel to a surface normal of sample surface, i.e., illumination takes place through vertically directed light. Detection of light scattered by the sample liquid also takes place along an axis parallel to a surface normal of the sample surface. Two light emitters are separated from two detectors by vertically aligned opaque partitions having horizontal slots, the slots being arranged within the sample liquid in immediate vicinity of the sample surface. The partitions serve to prevent any repeated reflections from the vessel's bottom and walls. Excess sample liquid overflows an edge of the vessel in immediate vicinity of the spot, at which the light is directed. One problem with such an apparatus is that the intensity of detected light will be very low, since light incidence direction and detection direction are antiparallel to each other. Another problem could be that the sample surface is not very stable because of the close proximity of the overflow-edge and the spot, at which the light is directed. But, according to said RU Patent 2,235,310, any kind of instabilities do not at all affect the measurement, because measured intensities (of both detectors, upon illumination with each light source) are evaluated in a special way.
In JP Patent Document 03-54436 a turbidimeter is disclosed, in which polarized light is obliquely directed at the surface of a sample liquid contained in a container. Scattered light thereupon emitted from the sample liquid is detected polarization-sensitively by a detector located outside the sample liquid, the detector comprising a polarizer. Between the sample liquid and said polarizer a cylinder is arranged for intercepting external light. The container has an edge, which is to be overflown by excess sample liquid.